1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing the same and, more particularly, to a semiconductor device having a microelectrode and a method of manufacturing the same.
2. Description of the Prior Art
Conventionally, in a process of forming the gate electrode of a GaAs FET (Field Effect Transistor), a technique of forming a gate electrode having a T-shaped section by filling an oxide film opening pattern with a metal film is used to reduce the gate length while lowering the gate resistance and the capacitance between the gate substrates.
The structure of the conventional semiconductor device in which the gate electrode is formed by burying an electrode, and the steps of the conventional manufacturing method will be described below.
FIGS. 1E and 2 show an example of the gate electrode structure of the conventional semiconductor device.
A Schottky metal film 76, a barrier metal film 77, and a low-resistance metal film 78 are sequentially formed from the lower side in a pattern opening portion having a width of 0.2 xcexcm. The Schottky metal film 76 is 100 nm thick, and the barrier metal film 77 is 2,000 nm thick on the flat portion except the pattern opening portion. On the bottom portion in the pattern opening portion, however, the Schottky metal film 76 is only 10 nm thick, and the barrier metal film 77 is only 20 nm thick.
The low-resistance metal film 78 extends into a gate vertical portion 82, so the distance to the Schottky interface between the Schottky metal film 76 and a substrate operation layer 72 is about 30 nm. This is because the metal film thickness decreases in the small pattern opening portion.
FIGS. 1A to 1E are sectional views showing the steps in the manufacture of the gate electrode of the conventional semiconductor device.
The gate oxide film 72 is formed on a semi-insulating substrate 71, and then, a first insulating film 73 is formed. A resist film 74 is applied onto the first insulating film 73 and subjected to exposure and development to form a pattern opening portion 80 (FIG. 1A).
The first insulating film 73 is selectively removed by anisotropic dry etching using the opening pattern of the resist film 74 as a mask, thereby forming a gate opening pattern. After this, the resist film 74 is removed (FIG. 1B).
A second insulating film 75 is formed on the entire surface of the first insulating film 73. Since the second insulating film 75 also forms on the side wall of the opening portion, the size of the opening portion is reduced (FIG. 1C).
The entire surface is etched back by dry etching to expose the gate oxide film 72, thereby forming the gate opening pattern (FIG. 1D).
The Schottky metal film 76, the barrier metal film 77, and the low-resistance metal film 78 are sequentially formed on the entire surface. The metal films 76, 77, and 78 are selectively removed by ion milling and dry etching using, as a mask, a resist pattern formed on the pattern opening portion, thereby obtaining a T-shaped gate electrode 81 (FIGS. 1E and 2). FIG. 1E is an enlarged view of a section taken along a line IExe2x80x94IE in FIG. 2.
In the conventional method, the barrier metal film having a sufficient thickness must be inserted between the Schottky metal film and the low-resistance metal film. However, since the barrier metal film cannot obtain a sufficient thickness in the pattern opening portion, the metal of the low-resistance metal film diffuses into the substrate through the Schottky interface. This degrades the Schottky characteristics, resulting in the problem of low reliability of the FET.
The reason for this is as follows. The thickness of the barrier metal film formed by sputtering or the like becomes smaller on the bottom portion in the gate opening portion having a trench structure than that on the flat portion other than the pattern opening portion because of the shielding effect from the metal film which has already been formed.
The present invention has been made in consideration of the above situation in the prior art, and has as its object to provide a semiconductor device which allows reduction of the probability of degradation in Schottky characteristics of a FET, decrease the number of processes, and forming a gate metal film using sputtering or deposition, and a method of manufacturing the same.
The above object is achieved by the following aspects of the present invention.
According to the first aspect of the present invention, there are provided a semiconductor device having a structure in which a gate electrode is formed by sequentially forming a Schottky metal film, a barrier metal film, and a low-resistance metal film from a lower side, wherein the Schottky metal film or barrier metal film has a gap in a lower gate vertical portion, the gap is closed at upper and lower portions, and the overlaying low-resistance metal film does not extend into the lower gate vertical portion, and a method of manufacturing the semiconductor device.
According to the second aspect of the present invention, there are provided a semiconductor device having a structure in which a gate electrode is formed by sequentially forming a Schottky metal film and a low-resistance metal film from a lower side, wherein the Schottky metal film has a gap in a lower gate vertical portion, the gap is closed at upper and lower portions, and the overlaying low-resistance metal film does not extend into the lower gate vertical portion, and a method of manufacturing the semiconductor device.
In the semiconductor device according to the first aspect of the present invention, in the gate electrode of the field effect transistor constituted by the Schottky metal film, the barrier metal film, and the low-resistance metal film, the gap surrounded by the Schottky metal film and the barrier metal film is present in the lower gate vertical portion, and the gap is closed at its upper portion by the barrier metal film. Therefore, the overlaying low-resistance metal film does not extend into the gate vertical portion.
In the semiconductor device according to the second aspect of the present invention, in the gate electrode of the field effect transistor constituted by the Schottky metal film and the low-resistance metal film, the gap surrounded by the Schottky metal film is present in the lower gate vertical portion, and the gap is closed at its upper portion by the Schottky metal film. Therefore, the barrier metal film and the overlaying low-resistance metal film do not enter the gate vertical portion.
The method of manufacturing the semiconductor device according to the first aspect of the present invention comprises the steps of forming an insulating film on a gate oxide film formed on a semi-insulating substrate, selectively dry-etching the insulating film using a resist pattern as a mask to form a pattern opening portion having a high aspect ratio, forming a Schottky metal film, forming a barrier metal film, and forming a low-resistance metal film.
In the step of filling the pattern opening portion having a vertical portion on the lower side with the metal film to form a gate electrode, the Schottky metal film is formed to have a uniform thickness on the side wall and bottom portions of the pattern opening portion. After this, the barrier metal film is formed to be particularly thick on the side wall portion of the pattern opening portion. With this process, the pattern opening portion is closed at its upper portion, thereby forming a barrier metal film having a gap in the gate vertical portion.
Alternatively, the Schottky metal film is formed to be particularly thick on the side wall portion of the pattern opening portion. With this process, the pattern opening portion is closed at its upper portion, thereby forming a Schottky metal film having a gap in the gate vertical portion.
Since a certain distance can be ensured between the GaAs substrate and the low-resistance metal film because of the gap in the barrier metal film or Schottky metal film, and the low-resistance metal can be prevented from diffusing into the Schottky interface, the characteristics of the FET are minimally degraded.
As is apparent from the above aspects, since the low-resistance metal film is separated from the Schottky interface, probability of degradation in Schottky characteristics of the FET due to diffusion of the low-resistance metal into the substrate can be reduced, resulting in an increase in reliability.
The barrier metal film maybe made thinner or may be omitted. For this reason, the process can be shortened to result in improved productivity.
Since metal filling corresponding to the high aspect ratio need not be performed, the conventional metal film forming technique, i.e., sputtering or deposition using an inexpensive apparatus can be used to form the gate electrode.
The above and many other objects, features and advantages of the present invention will become manifest to those skilled in the art upon making reference to the following detailed description and accompanying drawings in which preferred embodiments incorporating the principles of the present invention are shown by way of illustrative examples.